Rate-Determining Step of the Reactions of Benzophenone with Various Grignard Reagents

Article abstract of DOI:10.1021/ja00195a052

Carbonyl carbon kinetic isotope effects (KIEs) have been determined for the reaction of benzophenone-7-(14)C with various Grignard reagents at 0 deg C.The observed KIEs are as follows: MeMgI/Et2O, ¹²k/¹⁴k = 1.056 +/- 0.002; MeMgBr/Et2O, 1.050 +/- 0.011; MeMgBr/THF, 1.056 +/- 0.004; PhMgBr/Et2O, 1.056 +/- 0.004; o-CH3C6H4MgBr/Et2O, 1.060 +/- 0.014; CH2=CHCH2MgBr/Et2O, 0.999 +/- 0.003; CH3CH=CHCH2MgBr/Et2O, 0.999 +/- 0.002; PhCH2MgBr/Et2O, 1.024 +/- 0.007; t-BuMgCl/Et2O, 1.010 +/- 0.007, 1.004 +/- 0.004; MeMgI + 5 mol percent FeCl3/Et2O, 1.063 +/- 0.033 (for 1,2-adduct formation), 0.997 +/- 0.019 (for pinacol formation).The relative reactivities of ortho-, meta-, and para-substituted benzophenones with these reagents were also determined by competition experiments.The reactions can be classified into three groups on the basis of these results: the first group gives a large (14)C KIE, a medium-sized Hammett ρ value, and a large steric rate retardation due to ortho substituents; the second gives a KIE of unity, a near-zero ρ value, and no steric rate retardation; and the third gives a small (14)C KIE, a large ρ value, and no steric rate retardation.It was concluded that the intimate mechanism of these Grignard reactions is diffrent although they all follow the single-electron-transfer (SET) scheme; specifically, the rate-determining step for MeMgX, ArMgBr, and PhCH2MgBr is C-C bond formation while that for allylic reagents is initial SET.The rate-determining step for t-BuMgCl is the isomerization of the radical ion pair intermediate.